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Integrating Dual Subwoofers with Single Driver Horn Loaded Speakers
by ct_acoustic_frontierJanuary 28, 2022
Cesar Rubio is a San Francisco Architectural photographer and keen audiophile. His tastes in components lean towards vinyl, valves and single driver or horn speakers. His high end audio system is installed in his living room which he also uses for entertaining.
This particular engagement involved integrating two JL Audio F112 subwoofers with a set of horn loaded single driver speakers from RL Acoustique. These speakers roll off very early considering their size - at about 80Hz.
[caption id="attachment_2663" align="aligncenter" width="225"] RL Acoustique speaker driver[/caption]
The subs were run off a second pre-amplifier output, which means the main speakers are run "full range". We do not normally recommend this approach because the only way to properly integrate subs with speakers run full range is to find the speaker roll off point and bring the sub in to fill in part of the frequency range below which the main speakers roll off. This severely limits the benefits of using subs to fill in speaker boundary interference dips and cancel room modes.
The other issue with running subs off Cesar's pre-amp output was that one sub was fed the left channel signal the other was fed the right channel signal. People call this "stereo" subs. If you run stereo subs then you can't use sub placement to cancel room mode resonances.
There are clearly significant downsides to running the main speakers full range and using stereo subwoofers, as we explain in this article on subwoofer setup in two channel systems. For that reason we advise actively rolling off the bass to the main speakers and combining the left and right signals to mono for the subwoofers.
Acoustic Frontiers Solution
Our System Setup service can include multiple things including speaker / listener positioning, equalization and subwoofer setup. This particular engagement just focused on the subwoofers.
We used a laptop based acoustic measurement system to dial in the following controls on the subwoofers:
Low pass frequency
Low pass slope
Extra low frequency extension. This particular control is only found on JL Fathom series subwoofers.
Automatic room optimization. This is a room correction system that is proprietary to JL. It applies a single band of EQ.
[caption id="attachment_2661" align="aligncenter" width="640"] Cesar's system with the two JL F112s.[/caption]
After measuring the response of the speakers without subs we progressively optimized the sub controls until the result below was achieved.
The frequency response is significantly improved, in particular the large resonance around 65Hz was reduced in amplitude relative to the surrounding frequencies.
Despite the dual subs there are clearly still some areas that could be improved such as the sub 30Hz response if additional equalization tools were available. Furthermore, if the subs were fed a single mono signal we could have used room mode cancellation techniques. Even with these constraints in terms of system topology, we were able to achieve a response that stays within a 6dB window from 40Hz through 150Hz.
[caption id="attachment_2660" align="aligncenter" width="700"] Frequency response, 1/12th octave, with and without subwoofers[/caption]
In the Client's Words
Cesar was also very happy with the work we had done:
"My audio evolution has led me to the world of low power SET and single driver, high efficiency speakers. It has rendered my musical experience with the most clarity and focus to date, which has had many rewards but some pitfalls, mainly in the realm of sound bellow 70hz, in other words, the bottom fell out.
My speaker is a back-loaded horn design by RL Acoustique using a driver made by AER, model MD3B. it can produce what is considered full-range in the right settings but my room is not ideal for pressurization. For one, it has a large opening to a hallway and a stairwell; secondly, there are single pane windows on one side, both of these factor lead to sound leakage; and lastly, the room is 16 x 23 feet with 9 foot ceilings.
After much consideration and research I came upon JL subs as an addition to my system, basically creating a 2-way out of a single driver. JL has a reputation for speed which is ideal since the AER is a very fast and accurate driver. Since all things in stereo are better in pairs I bought 2 JL F-112 subs which have a myriad of controls; looking at the panel was a little intimidating but not insurmountable.
After a few days of monkeying around I decided to get help, it didn’t sound bad but it also didn’t sound right. The good people at Music Lovers Audio recommended Nyal from Acoustic Frontiers; he was tuning the main listening room at the San Francisco store at the time.
He came to my place armed with a laptop, mics, cables and a laser measuring devise for the diagnostic as well as samples of room treatments options. The process was interesting and informative, kind of an acoustical lesson really. We moved the subs and the mains a few times, took readings, examined the graphs and adjusted the cross-over settings. After a few hours we were done.
All the changes made to the cross-overs and phase controls were marked so if I wanted to experiment I could always go back and re-establish the original settings. This way I had freedom to experiment, set new levels for action movies, etc. Nyal also provided a report which had all the readings and recommended acoustical treatments, some of which I have implemented and some I can’t afford quite yet.The sound in the room has changed considerably; not only is the center image much stronger but also the soundstage is much wider and more encompassing. The subs are integrated perfectly and have illuminated the pace and rhythmic quality of the music...I was missing much more than I had ever imagined.Long ago, a friend in the world of audio had expressed the importance of the room; to paraphrase, the room is the speaker since all the sound is coalesced and interacting with the six surfaces which make up the listening environment. There is no perfect room but one can, with some help from Nyal at Acoustic Frontiers, get much closer to a better place." Mr. Rubio, San Francisco, CA
May's project of the month is a dedicated listening room incorporating YG Anat speakers and BMC amplifiers and DACs. Acoustic Frontiers determined speaker and listener positions, designed and provided the acoustic treatment scheme and calibrated the EQ.